FIELD OF THE INVENTION
The invention relates to a method for operating a gas-turbine and steam-turbine plant, in which oxygenous gas from a gas turbine is utilized for steam generation, a first part stream of exhaust gas from the gas turbine is used as combustion air for the combustion of a fossil fuel, a second part stream of exhaust gas from the gas turbine is utilized for waste-heat-steam generation, the steam generation by the combustion of the fossil fuel and the waste-heat steam generation take place in a common water/steam circuit of the steam turbine, and feedwater of the water/steam circuit is preheated in part streams.
The invention also relates to a gas-turbine and steam-turbine plant working according to the method, including a fossil fired steam generator which is connected into a water/steam circuit of a steam turbine and to which a waste-heat steam generator is connected in parallel on the water/steam side, both the fired steam generator, through a first part-stream conduit, and the waste-heat steam generator, through a second part-stream conduit, are connected downstream of the gas turbine on the exhaust gas side.
Such a method and such a plant are known from German Patent DE 38 15 536 C1 and U.S. Pat. No. 4,852,344.
In the combination of a steam-turbine process and a gas-turbine process, there are in principle two possibilities for utilizing the exhaust gas from the gas turbine for steam generation. As is described in the paper entitled "Kombinierte Gas-/Dampfturbinenprozesse" Combined Gas-/Steam-Turbine Processes! in Brennstoff-Warme-Kraft Fuel/Heat/Power! 31 (1979), No. 5, May, in a possible combined process with a downstream steam generator, the oxygen-rich exhaust gases of the gas turbine serve as combustion air for the fossil-fired steam generator. In another combined process with a downstream waste-heat steam generator, the gas-turbine and steam-turbine processes are combined by utilizing the waste heat of the gas turbine in the waste-heat steam generator. A gas-turbine and steam-turbine power station with a waste-heat steam generator and a solar-heated steam generator and with a fossil-heated heat exchanger downstream of an additional combustion chamber, is known from German Published, Non-Prosecuted Patent Application DE-OS 41 26 036.
In a combined process, the powers of the steam turbine and gas turbine and of the fired steam generator are dependent- on one another, so that when a plant of that type is constructed, they have to be coordinated with one another. That applies not only to a retrofitting of an already existing steam-turbine plant, but also to a new plant. The coordination is usually carried out in such a way that, in the nominal load operating mode, the oxygen requirement of the fired steam generator can be covered by the exhaust gases of the gas turbine. However, gas turbines with only a few different power ratings, for example with 50 MW, 150 MW or 200 MW, are manufactured and offered, so that it is extremely difficult to adapt them to the power of the steam turbine and to that of the steam generator. Consequently, for a predetermined plant size, in the full-load range the gas turbine supplies either too large or too small an exhaust-gas quantity in comparison with the exhaust-gas quantity required as combustion air for the fired steam generator. If the exhaust-gas quantity is too small, only a low efficiency of the plant is to be achieved in the full-load range, and that then becomes better in the part-load range.
In contrast, the result of too large an exhaust gas quantity from the gas turbine can be that, in a combined process in which the excess exhaust gases from the gas turbine are guided past a combustion chamber of the fired steam generator to a boiler preheater or feedwater preheater (economizer), the latter already experiences evaporation in an undesirable way due to the excessively high introduction of heat. Or, if there is too large an exhaust-gas quantity in the part-load range, the power of the gas turbine already has to be reduced at an early moment. However, with an increasing reduction in the power of the gas turbine, the efficiency of the plant in the part-load range decreases. In other words, in both cases, the overall efficiency that is achieved is limited. Therefore, particularly during the retrofitting of an already existing steam-turbine plant, a power increase arising from the gas turbine has to be dispensed with if the exhaust-gas heat of the gas turbine cannot be fully utilized or an acceptable part-load behavior cannot be obtained.
In contrast to the combined process with a downstream fired steam generator, the combined process with a downstream waste-heat steam generator is particularly suitable for the retrofitting of an already existing gas-turbine plant. In the case of a new plant, usually a number of gas turbines having a corresponding number of waste-heat steam generators are connected to a common steam turbine. Since, in that combined process, the steam generation is restricted to a pure waste-heat utilization, the overall efficiency of the plant is likewise limited. Furthermore, in that combined process as well, there is the problem of finding a suitable gas-turbine model in the event of a necessary or desired exchange of the gas turbine for a gas turbine of comparatively high power. That is because, for a predetermined power of the steam turbine and consequently a predetermined rating of the waste-heat steam generator, the introduction of heat through the use of the exhaust gas from a comparatively large gas turbine into the waste-heat steam generator would be too high, so that, particularly in preheaters (economizers) disposed within the steam generator, evaporation would already take place in an undesirable way.